Field of the Invention
This invention relates to an exposure apparatus for the manufacture of semiconductors which uses as an exposure light source a laser such as an excimer laser requiring gas replacement, gas charge or the like.
Related Background Art
In recent years, an increase in the degree of integration of semiconductive integrated circuits has led to smaller minimum pattern dimensions of the circuit and therefore, there has been developed an exposure apparatus using an excimer laser instead of the mercury lamp which has heretofore been the standard light source. The excimer laser exposure apparatus is generally comprised of an excimer laser source and an exposure apparatus body, and as the exposure apparatus body, the reduction projection type successive movement system, i.e., the so-called stepper, is nowadays conventional because of its excellence in resolving power and mask manufacturing property. A popular excimer laser exposure apparatus is one in which the excimer laser and the exposure apparatus body are coupled together by an electric wire or an interface cable of optical fiber and the laser is caused to emit light in accordance with the sequence of a main computer in the exposure apparatus body. As the signal of the interface, mention may be made, for example, of a signal from the exposure apparatus body to the excimer laser which is representative of light emission trigger, starting of high voltage charging, starting of oscillation, stoppage of oscillation, etc., or a signal from the excimer laser to the exposure apparatus body which is representative of completion of oscillation standby, internal shutter position, interlocking operation, etc.
The excimer laser generally uses mixed gas comprising three kinds of gases, i.e., halogen gas such as fluorine, inert gas such as krypton or argon, and rare gas such as helium or neon is enveloped in a laser chamber and the halogen gas and the inert gas react to cause a discharge in the chamber to thereby emit laser light (a light pulse of nsec order. However, while the emission of the laser light is repeated, the halogen gas is coupled to impurities created in the chamber or is adsorbed to the inner side of the chamber and therefore, the concentration of the halogen gas is reduced and the pulse energy of the laser is reduced. Where the excimer laser is used as the light source of a semiconductor exposure apparatus, any fluctuation of pulse energy results in the following problems:
(1) The control accuracy of energy (exposure amount) arriving at an object to be exposed (such as a wafer) is reduced;
(2) The function of reducing the interference fringes on the object to be exposed which are attributable to an optical system is reduced; and
(3) The S/N ratio of the signal of the photoelectric detecting system of a pulse energy monitoring system or an alignment system is reduced.
Therefore, the excimer laser is designed such that the pulse energy reduced by a reduction in the concentration of the halogen gas is monitored and fed-back to an applied voltage for discharge and the applied voltage for discharge is gradually heightened, thereby keeping the pulse energy constant. However, there is an upper limit in the applied voltage for discharge and therefore, it has been necessary that when the applied voltage reaches the upper limit, the HI (halogen injection) operation be performed to restore the concentration of the halogen gas to a proper value and along therewith, the applied voltage be reduced to keep the pulse energy constant.
The state of this HI operation is shown in FIG. 3 of the accompanying drawings.
FIG. 3(A) is a graph in which the pulse energy emitted from the excimer laser is plotted as the vertical axis and time t is plotted as the horizontal axis, and FIG. 3(B) is a graph in which the applied voltage for discharge to an electrode in the laser chamber is plotted as the vertical axis and time t is plotted as the horizontal axis. As shown in FIG. 3(A), when the allowable upper value and the allowable lower value are determined, about the set value of the pulse energy required on the exposure apparatus side, the magnitude of the excimer laser source pulse energy is compared with the set value for each pulse by the use of an energy monitor (such as a light receiving element provided within the excimer laser source), and when the pulse energy begins to be reduced, the applied voltage for discharge is gradually increased as shown in FIG. 3(B). The applied voltage for discharge also has upper value and a lower value determined therefor, and the actual voltage range thereof differs depending on the internal structure of the excimer laser source, the maker, etc. Now, when at time t.sub.1, the applied voltage for discharge reaches the upper value, a control processor provided within the excimer laser source judges that the HI operation is necessary, and injects a predetermined quantity of halogen gas into the laser chamber. Immediately after the injection, the concentration of the halogen gas restored to its original level, but the applied voltage for discharge cannot be suddenly restored (reduced) to its original level. At time t.sub.2 thereafter, the HI operation is likewise performed.
The reason why the applied voltage for discharge must be gradually reduced is that immediately after the HI operation, the gases in the laser chamber cannot be said to be mixed sufficiently uniformly and the possibility of the pulse energy becoming irregular is very great and therefore, if the applied voltage for discharge is suddenly reduced immediately after the HI operation, no pulse light will be oscillated and this may result in the problem that even power monitoring cannot be accomplished.
When the halogen injection is repeated, impurities in the laser chamber begin to increase.
If the injection of halogen gas is effected with such an increase in the impurities, the halogen gas is coupled to these impurities (the concentration of the halogen gas is reduced) and the applied voltage for discharge for keeping the pulse energy constant rises. Thereby, the period of the halogen gas injection becomes shorter and soon, even if the halogen gas injection is effected, it will become impossible to keep the pulse energy constant.
FIGS. 4(A) and (B) of the accompanying drawings show that state, and correspond to the graphs of FIG. 3. As shown in FIG. 4(B), the injection period becomes gradually shorter at times t.sub.1, t.sub.2, . . . , t.sub.6, and after the time t.sub.6, in spite of the applied voltage for discharge being at the upper value, the pulse energy is gradually reduced.
When the effect of the halogen gas injection has thus become nil or when it has been reduced to a predetermined condition, it has been necessary to partially replace the aforementioned mixed gas comprising three kinds of gases, i.e., execute the PGR (partial gas replacement) operation, thereby maintaining the pulse energy. An example of the state during the execution of the partial gas replacement is shown in FIG. 5 of the accompanying drawings. The state of variations in the pulse energy and the applied voltage for discharge when the partial gas replacement is effected along with FIG. 5 will be described below.
FIGS. 5(A) and (B) correspond to FIGS. 4(A) and (B), respectively, and times t.sub.3, t.sub.4 and t.sub.5 in FIGS. 5(A) and (B) are the same as those in FIG. 4 and represent the timings of the HI operation. At time Ta after the time t.sub.5, the PGR operation is executed.
At this time, the applied voltage for discharge has already come near the upper value as previously described and therefore, it is difficult to increase the applied voltage and restore the pulse energy to its set value. Therefore, a predetermined quantity of new mixed gas is injected into the laser chamber. By this injection, the pulse energy is increased and substantially exceeds the set value (exceeds the allowable upper value) and therefore, the applied voltage is gradually reduced to thereby restore the pulse energy to the set value. Thereafter, as in FIG. 3, the halogen injection (HI) operation is repeated until the partial gas replacement becomes necessary again.
The reason why the applied voltage is gradually reduced during this PGR replacement operation is also the same as that during the aforedescribed HI operation.
The above-described HI operation and PGR operation have been substantially automatically performed by the command of a control processor in the excimer laser source.
When the excimer laser source is viewed from the exposure apparatus body side, it is desirable that the pulse energy be constant as previously described, but it is difficult to make the pulse energy completely constant, because of such factors as the accuracy of control of pulse energy, the above-described halogen injection and the partial gas replacement. Therefore, on the exposure apparatus body side, the allowable value of pulse energy fluctuation for satisfying satisfactory operation is set, and various contrivances are used in order that on the excimer laser side, the pulse energy may be within the set value.
One technique therefor is the HI operation and the PGR operation described with reference to FIGS. 3 to 5, but on the exposure apparatus body side, there has been a problem that in the intervals indicated by times a and b in FIG. 5(A), the pulse energy exceeds the allowable value. These times a and b differ depending on the structure of the laser source, the maker, etc., and are of the order of several seconds to several minutes at present. Further, a serious disadvantage to the exposure apparatus side is that the intervals a and b, i.e., during the HI operation and the PGR operation, have been solely controlled by the laser source side.
Thus, if the HI operation or the PGR operation is effected non-synchronously during the exposure of one shot area (which usually requires tens pulses) when a wafer is to be exposed by the step and repeat system, great damage will be imparted to the exposure of almost all of many shots after that shot area, or the next shot area on the wafer.
Of course, when the excimer laser light is used for other purpose, for example, for the relative alignment of a reticle (negative) and a wafer, if the HI operation and the PGR operation take place during the alignment period, there occurs an alignment error.